Large scale tropospheric warming of the Northern and Southern Hemisphere
extratropics occurred during the period January 1998-January 2000. The
anomalies were remarkable for their amplitude, their persistence, their zonal
symmetry with warming occurring at all longitudes between 30-50 degrees
latitude, and their axial symmetry with respect to the equator. The origin
of the anomalies are investigated using atmospheric general circulation models
forced by the global sea surface temperatures (SSTs) of this period. Two
ensembles, based on different models, reproduce the key observed features
indicating that the unusual climate state was forced by the global SSTs.
However, the atmospheric states were unrelated to the strong and protracted
La Niña of the east equatorial Pacific Ocean during this time. We
speculate on the role of the Indo-Pacific warm pool, which itself experienced
unprecedented warming in 1998 and 1999.